Life on Mars?

The first method follows the principle that when you’re looking for something, it’s best to start close to home.

We know of one planet that has life: Earth. So it makes sense to look for other places with Earth-like conditions: that is, liquid water, oxygen in the air, nutrients for growth, and so on.

The most obvious place to look is Mars. At first glance it appears dry, cold and dead. But if you can see past that, things start to look up. The polar caps, for example, have lots of frozen water, and we’ve directly seen ice at lower latitudes on the Red Planet as well – meteorite impacts have left behind shiny craters, digging up fresh ice from below the surface.

Several Mars rovers and landers have uncovered tantalising evidence that liquid water might flow just beneath the surface, but we still lack any conclusive evidence. However, if you broaden your timescale a bit, there is excellent evidence that in the past – perhaps a billion years or so ago – our neighbouring planet had oceans of liquid water and thicker air. In fact, conditions were pretty good to develop life as we know it even before it popped up here on Earth.

It’s entirely possible that life got a toehold (or pseudopod hold) there long ago, and died out. If that’s the case, we may yet find fossils in the Martian rocks. Again, there’s no conclusive evidence yet, but we’ve literally barely scratched the surface there. Now that it has successfully landed on Mars, we have the exciting possibility that the plutonium-powered, car-sized Curiosity rover will soon use its on-board laser and other tools to crack open and examine rocks in the Gale Crater, which were laid down billions of years ago in the presence of liquid water.

And Mars isn’t the only possibility in our solar system. Liquid water exists inside Saturn’s moon Enceladus, where geysers of liquid water erupt from deep canyons at its south pole. Energised by the gravitational tug of the giant ringed planet itself, the interior of Enceladus may be a vast ocean of liquid water even while the surface is frozen over. That doesn’t guarantee we’ll ever find alien fish swimming that moon’s seas, of course. But it’s an interesting place to look.

Europa, a moon of Jupiter, almost certainly has an undersurface ocean as well. If you relax your constraints even more, Saturn’s moon Titan has lakes of liquid methane and ethane on its surface, too. The chemistry for life would be different there – it’s a rather chilly -180C on the surface – but it’s not impossible to suppose life might arise there too.

Finding out whether this is the case means getting up close and personal. We’re doing that for Mars; however, the likes of Europa and Enceladus may have to wait a decade or four.